The present invention relates to tungsten deposition processes and the resulting tungsten layers to form contact plugs in integrated circuit (IC) devices. More particularly, the present invention provides tungsten contact plugs that are substantially free of "seam" formations.
A tungsten contact plug is widely used in the art to provide electrical connection between conductive and/or semiconductive layers in an IC. Conventional processes to form such tungsten plugs include applying a dielectric layer, such as silicon dioxide (SiO.sub.2), to a substrate surface that may include polysilicon, silicon (Si) or metal, e.g., aluminum or tungsten. A pattern is then formed on the dielectric layer by a standard photoresist method, and a contact hole or a "via" is etched into the dielectric layer.
FIGS. 1A-1D show a conventional method for transforming such a via into a tungsten contact plug. FIG. 1A shows a via 16 that is formed inside a SiO.sub.2 layer 12, which is disposed above a silicon substrate 14, for example. A titanium layer 10 is blanket deposited over the entire surface, as shown, partially filling via 16. The titanium layer, due to its high-electrical conductivity, ensures a good ohmic contact with the underlying silicon substrate. In other applications, the titanium layer may ensure an ohmic contact with such underlying layers such as a metallization layer or a polysilicon layer.
As shown in FIG. 1B, a layer of titanium nitride (TiN) 18 is typically blanket deposited over via 16 and dielectric layer 12 of FIG. 1A by means of sputtering or chemical vapor deposition. Titanium nitride (TiN) layer 18 acts as a barrier layer to prevent diffusion of fluoride atoms from getting into the substrate. The fluoride atoms are typically produced in subsequent chemical vapor deposition steps that involve depositing tungsten into the vias. TiN layer 18 also facilitates uniform nucleation of tungsten grains, which grow at sidewalls and the bottom of via 16, and thereby promotes adhesion of the bulk tungsten layer to titanium and SiO.sub.2 layers disposed below TiN layer 18.
Next, as shown in FIG. 1C a nucleation layer 20 of tungsten is blanket deposited over the partially filled via structure of FIG. 1B typically by chemical vapor deposition. The chemical vapor deposition process, in this step, is conducted in the presence of silane (SiH.sub.4) and tungsten fluoride (WF.sub.6), where a flow ratio of silane (SiH.sub.4) and tungsten fluoride (WF.sub.6) gases into the deposition chamber is maintained at a value that is below about 1. Here the more readily reduced SiH.sub.4 chemistry is desirable as it is less aggressive on the substrate and yields a shorter nucleation time. Under these conditions, nucleation layer 20 is formed over the sidewalls and bottom of via 16. Deposition of nucleation layer 20 continues until a polycrystal layer of tungsten is deposited over TiN layer 18. For more information on this process, reference may be made to M. Iwasaki, H. Itoh, T. Katayama, K. Tsukamoto and Y. Akasaka, Tungsten Workshop V, (1990), which is incorporated herein by reference for all purposes.
After tungsten nucleation layer 20 is formed, bulk deposition of tungsten begins. FIG. 1D shows that a bulk layer 22 of tungsten is blanket deposited over nucleation layer 20 by chemical vapor deposition. A bulk layer 22 of tungsten is produced, in this step, from WF.sub.6 in the presence of hydrogen gas, which reduces WF.sub.6 to produce tungsten and hydrogen fluoride (HF) gas that dissipates easily. The bulk deposition of tungsten proceeds until via 16 is completely filled or totally closed. Chemimechanical polishing or plasma assisted etching, is then performed to remove the residual layers deposited on the open surface of dielectric layer 12 to form a contact plug.
Referring back to the bulk deposition process, most current processes for filling of contact holes or vias employ a blanket tungsten deposition, which is characterized by isotropic tungsten grain growth, i.e., tungsten grains grow on all areas of the via including the inner sidewalls and bottom of the via. The blanket deposition of bulk layer 22, as shown in FIG. 1D, accordingly results in grain growth on all areas inside via 16, particularly on the inner side-walls of via 16. Note that tungsten grains grow as columnar structures away from the side and bottom walls of via 16. As the tungsten grains continue to grow from the side-walls of via 16, they eventually physically contact one another at the center of via 16. By the time tungsten grains cease to grow, bulk layer 22 has a large mean tungsten grain size, especially on the inner side-walls of via 16.
Unfortunately, the large mean tungsten grain size creates a pronounced porous "seam" 24 at about a mid-region of via 16, as shown in FIG. 1D. Seam 24 is undesirable because it provides easy access to corrosive liquids that are employed in subsequent IC wafer fabrication steps, for example, hydrogen peroxide (H.sub.2 O.sub.2), potassium hydroxide (KOH), ferric compounds such as ferric nitrate (Fe(NO.sub.3).sub.2) and the like commonly employed in the chemi-mechanical polishing process mentioned above. Of course, seam 24 would also provide an easy diffusion pathway for other undesirable particles and compounds to diffuse into the contact plug. Once these undesirable compounds find their way into the contact plug, they may react with the tungsten inside the contact plug at subsequent high temperature processes. Consequently, the depletion of tungsten can reduce the electrical conductivity of the contact plug significantly, disabling any electrical connection made by the via, and thereby rendering the entire IC device inoperable. This translates into a substantially lower IC yield.
What is needed is a tungsten contact plug that is substantially free of seams or other pathway to allow undesirable particles and compounds to diffuse into the contact plug.